Massimo Sisti

Ternary Gd(III)L-HSA adducts: Evidence for the replacement of inner-sphere water molecules by coordinating groups of the protein. Implications for the design of contrast agents for MRI

Abstract Two novel gadolinium(III) chelates based on the structure of the heptadentate macrocyclic 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) ligand have been synthesized and their relaxometric and luminescent properties thoroughly investigated. They contain two water molecules in the inner coordination sphere in fast exchange with the bulk solvent and bear either a p-bromobenzyl or a p-phosphonatomethylbenzanilido substituent for promoting further interaction with macromolecular substrates. Upon interaction with human serum albumin the expected relaxation enhancement is not observed owing to displacement of the two inner-sphere water molecules of the complexes by a donor atom (likely from a carboxylate group) on the protein and possibly the phosphate anion of the buffered solution, respectively. We modeled the observed behavior by measuring the decrease of the relaxation rate of the water protons upon addition of malonate anion to aqueous solutions of the complexes. Conversely, no change in the hydratation state of the Gd(III) center for both complexes has been observed when the substrate for the formation of the macromolecular adduct is represented by poly-β-cyclodextrin.

NMR relaxometric studies of Gd(III) complexes with heptadentate macrocyclic ligands

The water 1H and 17O NMR relaxation properties of solutions containing Gd(III) chelates of the heptadentate DO3A, PCTA[12] and PCTP[12] ligands were thoroughly investigated and the results obtained are compared with those previously reported for other Gd(III) complexes with octadentate ligands {H3DO3A=1,4,7,10‐tetraazacyclododecane 1,4,7‐triacetic acid; H3PCTA[12]=3,6,9,15‐tetraazabicyclo[9.3.1]pentadeca‐1(15),11,13‐triene‐3,6,9‐triacetic acid; H6PCTP[12]=3,6,9,15‐tetraazabicyclo[9.3.1]pentadeca‐1(15),11,13‐triene‐3,6,9‐tris(methanephosphonic) acid}. The observed behaviour is consistent with a hydration number q=2 in the case of GdDO3A and GdPCTA[12] and q=1 in the case of PCTP[12]. The high relaxivity of the latter complex is accounted for in terms of the occurrence of an additional contribution arising from water molecules tightly bound to the phosphonate moieties on the surface of the paramagnetic chelate. Furthermore, it was found that the decreased relaxation rates observed at basic pH in the case of GdDO3A and GdPCTA[12] can probably be ascribed to a partial decrease in their hydration. The measurement of 17O NMR transverse relaxation rates, in the temperature range 273–342 K, allowed the assessment of the water exchange rate between the coordination site and the bulk solvent. A particularly short exchange lifetime was measured for the octacoordinate GdPCTP[12], which suggests the occurrence of an associative exchange mechanism. Further insights into the understanding of the structural properties of the three complexes were gained by measuring the magnetic field dependence (NMRD profiles) of the proton relaxivity on a Koenig–Brown field cycling relaxometer.

NOVEL PARAMAGNETIC MACROMOLECULAR COMPLEXES DERIVED FROM THE LINKAGE OF A MACROCYCLIC Gd(III) COMPLEX TO POLYAMINO ACIDS THROUGH A SQUARIC ACID MOIETY

Macromolecular Gd(III) complexes may find useful application as contrast agents for magnetic resonance angiography (MRA). Herein two novel systems are reported, namely Gd(DO3ASQ)3-lys16 and Gd(DO3ASQ)30-orn114. Their syntheses are based on the ability of the squaric acid moiety to act as a linker between the DO3A (1,4,7, 10-tetraazacyclododecane-1,4,7-triacetic acid) chelate moiety and the polyamino acidic chain. Moreover, the squaric acid participates in the coordination cage of the Gd(III) ion. The investigation of 1H and 17O NMR relaxation processes of solvent water nuclei allowed a detailed characterization of the systems under study. Gd(DO3ASQ)30-orn114 displays a remarkable ability to enhance the water proton relaxation rate of its solutions, and it may be considered as potential contrast agent for MRA applications.

Non-covalent conjugates between cationic polyamino acids and GdIII chelates: a route for seeking accumulation of MRI-contrast agents at tumor targeting sites.

Three novel Gd chelates containing on their external surface pendant phosphonate and carboxylate groups, which promote the interaction with the positively charged groups of polyornithine and polyarginine, have been synthesized. Their solution structures have been assessed on the basis of 1H- and 31P-NMR spectra of the Eu and Yb analogues. A thorough investigation of the relaxometric (1H and 17O) properties of the Gd chelates has been carried out and the observed relaxivities have been accounted for the sum of three contributions arising from water molecules in the first, second, and outer coordination layers, respectively. It has been found that the occurrence of a tight second coordination coating renders the dissociation of the water molecule directly coordinated to the Gd ion more difficult. The binding interactions between the negatively charged Gd chelates and the positively charged groups of polyornithine (ca. 140 residues) and polyarginine (ca. 204 residues) have been evaluated by means of the proton relaxation enhancement (PRE) method. Although the binding interaction decreases markedly in the presence of competitive anions in the solution medium, the affinity is strong enough that in blood serum it is possible to meet the conditions where most of the chelate is bound to the polyamino acid substrate. On this basis one may envisage a novel route for a MRI location of tumors as it is known that positively charged polyamino acids selectively bind to tumors having a greater negative charge than non-tumor cells.

Towards MRI contrast agents of improved efficacy. NMR relaxometric investigations of the binding interaction to HSA of a novel heptadentate macrocyclic triphosphonate Gd(III)-complex

Abstract A novel heptacoordinating ligand consisting of a thirteen-membered tetraazamacrocycle containing the pyridine ring and bearing three methylenephosphonate groups (PCTP-[13]) has been synthesized. Its Gd(III) complex displays a remarkably high longitudinal water proton relaxivity (7.7 mM–1 s–1 at 25  °C, 20 MHz and pH 7.5) which has been accounted for in terms of contributions arising from (1) one water molecule bound to the metal ion, (2) hydrogen-bonded water molecules in the second coordination sphere, or (3) water molecules diffusing near the paramagnetic chelate. Variable-temperature 17O-NMR transverse relaxation data indicate that the residence lifetime of the metal-bound water molecule is very short (8.0 ns at 25  °C) with respect to the Gd(III) complexes currently considered as contrast agents for magnetic resonance imaging. Furthermore, GdPCTP-[13] interacts with human serum albumin (HSA), likely through electrostatic forces. By comparing water proton relaxivity data for the GdPCTP-[13]-HSA adduct, measured as a function of temperature and magnetic field strength, with those for the analogous adduct with GdDOTP (a twelve-membered tetraaza macrocyclic tetramethylenephosphonate complex lacking a metal-bound water molecule), it has been possible to propose a general picture accounting for the main determinants of the relaxation enhancement observed when a paramagnetic Gd(III) complex is bound to HSA. Basically, the relaxation enhancement in these systems arises from (1) water molecules in the hydration shell of the macromolecule and protein exchangeable protons which lie close to the interaction site of the paramagnetic complex and (2) the metal bound water molecule(s). As far as the latter contribution is concerned, the interaction with the protein causes an elongation of the residence lifetime of the metal-bound water molecule, which limits, to some extent, the potential relaxivity enhancement expected upon the binding of the paramagnetic complex to HSA.

Oxindole alkaloids. A novel non-biomimetic entry to (−)-Horsfiline.

Abstract A novel non-biomimetic synthesis of horsfiline has been developed. The key pyrrolidine forming reaction is the 1,3-dipolar cycloaddition of the thermally generated N -methylazomethine ylide to a suitable 3-alkylidene-indolin-2(3 H )one. The same strategy was also applied to the synthesis of pure ( R )-(−)-enantiomer.

A straightforward entry into enantiomerically enriched β-amino-α-hydroxyphosphonic acid derivatives

Abstract The asymmetric aminohydroxylation (AA) reaction of β-substituted vinylphosphonates under Sharpless protocol followed by hydrolysis afforded β-amino-α-hydroxyphosphonic acids in moderate to good ee.

Synthesis of Indole Derivatives with Biological Activity by Reactions Between Unsaturated Hydrocarbons and N-Aromatic Precursors

The present review is devoted to illustrate the state of the art of the syntheses of indoles, focusing particularly on the most recent developments of new synthetic approaches. Emphasis is given to the preparation of natural products or bioactive compounds containing the indole unit. We present a historical perspective of indole synthesis showing the strategies by choosing the nitrogen precursors. The review is organized sharing the indole synthetic approaches by using different nitrogen-containing functional groups in aromatic substrates used as source of the nitrogen of the indole moiety. Some functional groups and some typical reactions are particularly stressed and highlighted because of the limited coverage given in previous reviews published on this topic. Other synthetic approaches more used and discussed in recent, complete and excellent reviews on the topic are summarized but the most recent published results are highlighted. Our interest is particularly focused on the indolization procedures and on the different methods used for the ring closure and no attention is given to modification of indole structures starting from molecules with a preformed indole unit. Intriguing indole syntheses are continually discovered and the importance that the scientific community gives to these new developments is connected with the strategic role of molecules containing the indole unit. Indoles are the class of heterocycles with more applications and extensive interest due to their biological and pharmacological activity.

New Insights for Pursuing High Relaxivity MRI Agents from Modelling the Binding Interaction of GdIII Chelates to HSA

It was recognized early on that the relaxivity of a Gd complex at 0.5–1 T can be strongly enhanced if its molecular correlation time is lengthened by linking it to a slowly moving macromolecule. 2] In this context, a huge amount of attention has been devoted in the past decade to the study of systems able to form noncovalent adducts with serum albumin, which also has the advantage of yielding systems that remain confined in the blood vessels. Theory foresees the attainment of relaxivities >100 mm 1 s 1 for macromolecular monoaquo Gd chelates characterized by a molecular reorientation time of 10–30 ns. 6] However, in spite of a number of investigated systems, relaxivities of such magnitude for Gd chelates bound to HSA have never been found. One major limiting factor has been recognized to be the occurrence of an insufficiently fast exchange rate of the coordinated water (tM). [7, 8]

Contrast Agents for Magnetic Resonance Imaging: A Novel Route to Enhanced Relaxivities Based on the Interaction of a GdIII Chelate with Poly-β-cyclodextrins

The formation of a macromolecular adduct between a suitably functionalized GdIII complex and an exogeneous substance such as a poly-β-cyclodextrin (see picture) forms the basis of the novel route to contrast agents for magnetic resonance imaging endowed with high relaxivity discussed herein.